In order to operate a so-called three-way catalytic converter arranged in the exhaust system of an internal combustion engine for eliminating the three major toxic component (CO, HC and NOx), the air-fuel ratio of the exhaust gas should be maintained near the theoretical (or stoichiometric) air-fuel ratio value. In order to maintain the theoretical air-fuel ratio, various apparatus have heretofore proposed, in both a carburetor type engine and a fuel injection type engine, for controlling the air-fuel ratio. Generally speaking, each of the known systems is provided with an oxygen concentration cell type sensor (so-called O.sub.2 sensor) arranged in the exhaust system of the engine for generation of an electrical signal indicating the air-fuel ratio of the exhaust gas, and with a comparator circuit adapted for providing two deviation signals (generally speaking logic signals "1" and "0"). One of the deviation signals indicates that the air-fuel ratio is decreasing (rich), whereas the other of the deviation signals indicates that air-fuel ratio is increasing (lean). In the carburetor type engine, the deviation signals are utilized for driving an actuator unit, (for example a supplementary fuel injection valve for controlling the amount of additive fuel supplied to the engine intake system or a secondary air valve for controlling the amount of secondary air supplied to the intake or exhaust system of the engine), so that the air-fuel ratio is maintained near the theoretical ratio. When the engine is of the fuel injection type, the deviation signals control the amount of the fuel injected to the intake system of the engine so that the air-fuel ratio is maintained near the theoretical ratio.
In the prior art air-fuel ratio control apparatus, a comparator unit is utilized for obtaining the above mentioned deviation signals, which comparator unit includes a first and a second input. The first input receives a signal from the O.sub.2 sensor, the voltage level of which is periodically changed between a maximum level and a minimum level in accordance with the air-fuel ratio. The second input receives a predetermined constant level signal located between the maximum and the minimum levels. Therefore, two logic signals "1" and "0" (or deviation signals) are obtained by comparing the predetermined constant level at the second input with the changed voltage level at the first input. When the voltage level of the air-fuel ratio signal is higher than the predetermined level, one of the deviation signals is obtained. When the voltage level of the air fuel-ratio signal is lower than the predetermined level, the other deviation signal obtained.
However, the prior art comparator circuit suffers from such a drawback that due to a delay inherent in the control system, the air-fuel ratio can not be quickly maintained near the theoretical ratio. As a result, the three-way catalytic converter does not effectively reduce the three major toxic components remaining in the exhaust gas.